Implantable ventricular cardiac defibrillators typically use lithium/silver vanadium oxide (Li/SVO) electrochemical cells as their power source. For the implantable medical device itself, it is preferable that the device be relatively small in size, quick in response to the patient's medical needs, promote long device service life, and the like. Therefore, when cells are built for implantable medical applications, special electrode assembly designs are needed to meet all of these requirements. Additionally, for cells powering cardiac defibrillators, a large electrode surface area is required to provide the needed power capability. An efficient cell package is also needed to achieve the highest capacity in the smallest volume.
In a conventional electrode assembly for Li/SVO cells, the cathode active material is pressed, coated or otherwise contacted to both sides of a foil or screen cathode current collector to provide the cathode electrode. Lithium as the anode active material in the form of a foil is pressed onto both sides of an anode current collector to form the anode electrode. The anode and the cathode electrodes are then placed against each other with one or two layers of intermediate separator material. The final electrode assembly is typically in the form of a prismatic plate design or a jellyroll design. An example of the conventional prismatic plate design is disclosed in U.S. Pat. No. 5,147,737 to Post et al. An example of a conventional jellyroll design is disclosed in U.S. Pat. No. 5,439,760 to Howard et al. To further illustrate this point, FIG. 1 shows a conventional prismatic or jellyroll electrode assembly. The electrode assembly 10 includes an anode electrode 12 and a cathode electrode 14 segregated from each other by separator sheets 16. The anode electrode comprises an anode active material 18, such as lithium, contacted to both sides of an anode current collector 20, such as of nickel. The cathode electrode 14 comprises a cathode active material 22 contacted to both sides of a cathode current collector 24, such as of titanium. The outermost winds or plates are of an anode electrode directly adjacent to a casing sidewall 26. Separator sheets 16 reside between the electrode units and the outermost anode electrode structures. Finally, the entire electrode assembly is contained in a polymeric insulator bag 28 that is then inserted into the casing 26.
The anode electrode is electrically connected to the conductive casing 26 as the negative electrode terminal. The cathode electrode is electrically connected to a terminal lead insulated from the casing by a glass-to-metal seal (not shown). The general structure of a glass-to-metal seal is well know by those skilled in the art and does not necessarily form a basis for the present invention.
Since the electrode assembly thickness is the sum of the thickness of each component, the fewer the non-active component layers, the more volume for the active components and, consequently, the higher the cell's volumetric capacity. Therefore, the packaging efficiency of the prior art electrode assembly can be further improved by having the casing wall serve as part of the anode current collector.